1. Field of the Invention
The present invention relates to an image processing apparatus for performing edge enhancement processing, and an image processing method.
2. Description of the Related Art
Conventionally, edge enhancement processing can increase the readability of a character image or the sharpness of a photo, thereby improving the image quality. For the edge enhancement processing, convolution calculation by a filter is generally used. An expected output is an image having a high contrast near an edge. More specifically, by converting a bright portion to have a higher brightness and a dark portion to have a lower brightness at a position where the bright portion and the dark portion contact each other, their boundary portion is enhanced, and thus the image looks sharper.
A number of techniques have been proposed as a technique of performing edge enhancement processing for a color image. For example, there is provided a technique of increasing the readability by determining black characters in a scanned document image, performing edge enhancement for portions determined as the black characters, and generating the black characters by black ink in printing.
Japanese Patent Laid-Open No. 2006-340144 describes a technique of performing edge enhancement processing by detecting the pixels of an edge portion within a character line drawing region, and generating a density gradient for respective pixels so that the density of a pixel of an inter-edge portion sandwiched between the detected pixels of the edge portions becomes lower than that of a pixel of the edge portion.
In the above conventional example, however, general edge enhancement processing may generate an unnatural boundary. If, for example, a character or line with a significantly high saturation is drawn in the background with a different color, the color of the edge of the character or line changes by edge enhancement. As a result, the contrast of a boundary portion increases but the portion has an unnatural edge. More specifically, a bright red character is edged with dark red. Such image often has a sufficient sharpness without performing strong edge enhancement processing. Furthermore, even if the amount of enhancement is suppressed, the image quality is often high enough.
To the contrary, if a dark character or line is drawn in the background with a low saturation such as a light gray, edge enhancement processing causes an unprinted portion in a portion where the light gray background is adjacent to the character. It is necessary to suppress the amount of enhancement for such image to prevent an unprinted portion.
If edge enhancement is suppressed so as not to produce an adverse effect, as described above, it is often difficult to obtain a sufficient amount of enhancement for an image in which an edge should be enhanced.